Atypical small acinar proliferation: biopsy artefact or distinct pathological entity?

To determine if atypical small acinar proliferation (ASAP) represents minimally sampled prostate cancer not fully evaluated on a biopsy or a distinct pathological entity, by examining prostates removed at radical cystectomy, as a finding of ASAP of the prostate on needle-core biopsy is closely associated with the detection of cancer on subsequent biopsy.

In all, 65 consecutive cystoprostatectomy specimens taken from June 1990 to March 2004 had prostatic material reviewed by one genitourinary pathologist (S.E.M.). The presence of high-grade prostatic intraepithelial neoplasia (HGPIN), ASAP, and adenocarcinoma was recorded. Foci of ASAP found in the absence of cancer were assessed with additional sectioning, high-molecular weight keratin (CK903), and alpha-methylacyl coenzyme A racemase (AMACR) immunohistochemistry.

In all, 24 of 65 specimens (37%) had adenocarcinoma. Of the 41 without cancer, 18 (44%) had neither HGPIN nor ASAP, 14 (34%) had HGPIN alone, three (7%) had ASAP alone (four foci), and six had both HGPIN and ASAP (15%). Two foci of ASAP were not present on any further sectioning. The remaining eight foci all lacked CK903 stain, indicating disruption of the basal cell layer. Of these eight, only five were present for the AMACR stain, all of which were positive. Two of these five developed into a lesion considered cancer on further sectioning.

ASAP identified in incidental prostates represented marginally sampled cancer in at least two of 10 foci assessed. The remainder could not be resolved as benign on further evaluation, and remained suspicious for malignancy.